1. Field of the Invention
The present invention relates to a technique for cleaning a reaction chamber for depositing a thin film.
2. Description of the Related Art
In processes for manufacturing various memories and logic circuits in large scale integrated circuits (131) and thin film transistor (TFT) elements for liquid crystal displays (LCD), a conductive or insulating thin film is formed on a semiconductor wafer or a glass substrate. Such a thin film is formed of a material containing silicon, such as crystalline silicon, polycrystalline silicon, amorphous silicon, silicon nitride, silicon oxide or silicon oxi-nitride. The thin film of such a silicon-containing material, particularly a thin film of amorphous silicon, is conventionally manufactured by a plasma-enhanced chemical vapor deposition (CVD) technique as one of various kinds of thin film forming techniques.
In applying such a plasma-enhanced CVD technique, one important objective is to improve the quality of the thin film deposited. One of the main causes detracting from the quality of the thin film is extraneous materials or impurities contaminating the thin film. An example of how extraneous material interferes with the process is when a chemical species remaining on the surface of plasma CVD hardware, for example the electrodes, the substrate holder and the inner wall of a reaction chamber (hereinafter collectively referred to as "component members"), becomes liberated and contaminates the growing film.
For example, a thin film for a PN junction is deposited on a substrate using a plasma CVD apparatus as described below. First, a mixture gas, prepared by adding a trivalent impurity such as B, Al, Ga or In (for example B.sub.2 H.sub.6 gas) to silane gas, is used to perform a plasma-enhanced CVD reaction. A P-type semiconductor layer is thereby formed. Then, another mixture gas, prepared by adding a pentavalent impurity such as P or As (for example phosphene (PH.sub.3), or arsine (AsH.sub.3)), to silane gas, is used to perform a plasma-enhanced CVD reaction on the P-type semiconductor layer. An N-type semiconductor layer is thereby formed. In this process, boron (B) remaining on the component members in the reaction chamber after the first CVD step for forming the P-type semiconductor layer is liberated during the second CVD step for forming the N-type semiconductor layer. If boron is mixed as an extraneous material into the thin film, the desired semiconductor characteristics in the thin film cannot be obtained. In this case, the electrical conductivity of the N-type semiconductor layer is reduced. Conventionally, to prevent such an extraneous material from contaminating the thin film, "in-situ cleaning techniques" are used for cleaning the component members in the reaction chamber after the plasma CVD process. In-situ cleaning techniques refer to a method of cleaning the internal component members of the reaction chamber in a self-cleaning manner by using plasma generated in the reaction chamber without exposing the component members to the atmosphere.
One of the most commonly-used in-situ cleaning techniques is based on a method of supplying, as a cleaning gas, a gas having high reactivity with the thin film material, for example a fluorine containing gas, and generating a plasma in the cleaning gas. The residual material attached to the component members is removed by etching with the cleaning gas plasma. Thus, the component members are cleaned by the cleaning gas plasma.
For example, an amorphous silicon thin film is deposited on a substrate in a plasma-enhanced CVD process. After the formation of the thin film, the substrate is moved out of the reaction chamber. Thereafter, a gas introduction system is operated to introduce a gas containing fluorine, for example carbon tetrafluoride (CF.sub.4) or nitrogen trifluoride (NF.sub.3). A plasma generator is operated in this fluorine containing gas to generate plasma. The residual material is removed by etching with the fluorine active species produced in the plasma. The removed residual material is expelled out of the reaction chamber through an exhaust system.
However, when such an in-situ cleaning is performed, the cleaning gas residue attaches to the internal component members of the reaction chamber and remains in the reaction chamber. Hence, another problem arises in that, during the next step in the plasma CVD process cleaning, gas residue having high reactivity becomes mixed in the thin film as an extraneous material. Further, the reaction for depositing the thin film is hindered due to the presence in the reaction chamber of the cleaning gas residue having high reactivity, for example a gas containing fluorine which has been liberated from the component members, thereby reducing the thin film deposition rate.
A treatment to be performed after the in-situ cleaning ("post treatment", hereinafter) has been proposed as a technique for solving the problems resulting from the cleaning gas residue. This post treatment is a process of introducing a gas which reacts with the cleaning gas residue to form a product having high volatility. For example, if the above-mentioned fluorine containing gas is used as the cleaning gas, then hydrogen gas is introduced to generate hydrogen gas plasma. Excited hydrogen generated in the hydrogen gas plasma reacts with the residual fluorine chemical species, for example elemental fluorine or a fluorine compound, to produce hydrogen fluoride (HF). Since hydrogen fluoride has a high vapor pressure, it is evacuated out of the reaction chamber through an exhaust system by a vacuum pump.
The above-described conventional post treatment method has a disadvantage in that the period of time required for such post treatment is considerably long.
For example, if fluorine chemical species are removed by excited hydrogen as described above, the distribution density of the plasma in the reaction chamber tends to be the highest in the space in between the electrodes and gradually decreases as the distance away from the electrodes increases. Therefore, if only hydrogen gas plasma is employed, several quarter hours are necessary to supply a sufficiently large amount of excited hydrogen to all component members in the reaction chamber. More particularly, the excited hydrogen cannot easily reach all of the chamber walls. The time period for the hydrogen gas post treatment also depends upon the amount of generation and the life of the excited hydrogen in the hydrogen gas plasma. A post treatment requiting such a long period of time is not suitable for a mass-production plasma CVD process.